The present invention relates in general to threshold detectors and specifically to such detectors having adaptive reference levels. More specifically, this invention relates to adaptive reference threshold detectors that generate logic level outputs whenever the input signal level drops below a predetermined level.
Many circuit applications require the generation of a signal logic level pulse whenever a signal change exceeds a preselected threshold level. Conventionally, the method used to generate this logic level pulse includes the use of a transistor biased "on" at low current level and having its base capacitively coupled to the input signal being monitored. A negative going input signal would shut off transistor current thereby producing a logic level output signal in the transistor's collector circuit.
The basic problem and limitation of this type of circuit arrangement is that switching level and threshold detector sensitivity vary from transistor to transistor as they are highly dependent upon the gain and other characteristics of the individual semiconductor device. It is therefore necessary to accurately control the characteristics of the transistors selected to obtain uniform operation. In addition, when resetting this conventional type threshold detector, its forward base bias current is increased, turning the collector current on again. This causes a relatively slow reset and in addition occasionally produces multiple digital output signals for a single input level change.